1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method in which a condition controlling process of controlling an image forming condition is executed based on a density detection result on a patch image.
2. Description of the Related Art
In image forming apparatuses, such as copier machines, printers and facsimile machines, to which electrophotographic techniques are applied, an image density of a toner image may change depending on a difference between individual apparatuses, a change with time, an environment surrounding the apparatus such as a temperature and a humidity level, etc. Against this background, various types of techniques have been proposed aiming at stabilization of an image density. These techniques include one which requires to form a small test image (patch image) on an image carrier for instance and optimize an image forming condition, such as a developing bias which influences a density of an image, based on a density of the patch image. According to this technique, predetermined toner images are formed on an image carrier while changing an image forming condition, and image densities of toner images are detected or image densities of toner images obtained by transferring these toner images onto other transfer member such as an intermediate transfer medium are detected on an assumption that these toner images are patch images. An image forming condition is adjusted such that these patch image densities will match with a predetermined target density, to thereby obtain a desired image density. The series of operations corresponds to the process of adjusting the image forming condition described above, i.e., the so-called condition controlling process.
While various types of techniques for measuring a patch image density (hereinafter referred to as “patch sensing techniques”) have been proposed, those which use optical means are most popular. In other words, light is irradiated upon a surface area within an image carrier or a transfer member which carries a patch image, an optical sensor receives light reflected or transmitted by this surface area and a patch image density is identified based on the amount of the received light.
When an image forming apparatus which adjusts an image forming condition based on a patch image density is to form a toner image having an excellent image quality by appropriately setting the image forming condition, an important issue to is how to accurately detect a density of a formed patch image. Despite this, the patch sensing techniques using an optical sensor mentioned above have the following problem.
That is, with respect to an image forming apparatus to which such a conventional patch sensing technique is applied, it is known that although an image forming condition is adjusted regularly so that a patch image density will be constant, i.e., an output from an optical sensor will be constant, densities of images formed on a final transfer material such as a paper and a film are not always constant. These density changes are created with time in accordance with an operating state of the apparatus such as a remaining toner amount within the apparatus. For instance, in the event that a large number of the same images are formed since attachment of a new cartridge housing toner to the apparatus, image densities of the images in some cases change gradually.
Further, the image forming apparatus described above executes the condition controlling process immediately after a power source of the apparatus has been just turned on or when the number of prints reaches a predetermined value. However, execution of the condition controlling process at such timing alone does not make it easy to perform image formation while always ensuring a stable image quality.